My daughter's developed a rash on her legs (red marks, white ring - raised). her knee has now swollen up and hurts. what could it be?

Consider HSP: Henoch-Schonlein Purpura causes a skin rash typically below waist, pain in the joints, and sometimes stomach pain. The rash looks like small bruises or small reddish-purple spots. It's usually on the buttocks, around the elbows and on the legs. HSP can also cause fever, nausea, vomiting and diarrhea. Symptoms can last for about 4-6 weeks. HSP usually gets better without any treatment.